>>>National is on Saturday - Boat Race is on Sunday
An excellent point, Puternut, with which it's impossible to argue! (I think that I'm probably still stuck in the 1960s when the Boat Race nearly always followed the Grand National on TV).
>>>National is on Channel 4 - Boat Race on BBC
That's probably irrelevant anyway (given the above) but broadcasters do sometimes deliberately choose to schedule programmes to avoid clashes with major events on other channels, so that they don't end up with a minuscule audience.
Even so, the BBC still has to fit the Boat Race in between other programming, such as live coverage of the Davis Cup. My first link above (from the official Boat Race website) shows that the BBC does have some influence over the timing.